UPDATE 2: The heat exchanger of Reactor 4's Spent Fuel Pool restarted, pool temperature 37.6 degrees Celsius. See my latest post. (The post also has the photos of Reactor 3's Spent Fuel Pool, from inside the pool.)
UPDATE: Matsumoto was speaking about the Reactor 4 Spent Fuel Pool, and mentioned the temperature. I took to mean the temperature of the SFP, but it is more likely that Matsumoto suddenly switched back to talking about the temperature of the Reactor 1 dry well. Sorry for the confusion. A lot of viewers apparently thought Matsumoto was talking about the Reactor SFP like I did.
So, it was the Reactor 1 dry well temperature that started to rise noticeably starting 4AM on April 13, 2012.
No word yet on exactly what the temperature is in the Reactor 4 SFP.
According to Matsumoto, the temperature of the Reactor 4 Spent Fuel Pool started to rise noticeably starting 4AM April 13, 2012.
From 8PM on April 12, 2012 to 3AM on April 13, 2012, the temperature remained 49.9 degrees Celsius, he said.
Matsumoto didn't say what the temperature was at the time of the press conference (11AM on April 13), and none of the reporters asked him about it, with the reporter from Asahi again pressing on the nitrogen injection system.
On checking the TEPCO's handout for the press on April 12, 2012,
Reactor 4 SFP temperature as of 11AM, 4/12/2012: 28 degrees Celsius
Reactor 4 SFP heat exchanger stopped at: 2:44PM, 4/12/2012
Expected rise in temperature: 0.5 degrees Celsius/hour
Oh wait... 21.9 degrees Celsius rise in temperature in 12 hours (from 3PM on April 12 at 28 degrees Celsius to 3AM on April 13 at 49.9 degrees Celsius) is more like 1.825 degrees Celsius per hour. But it is possible that the temperature doesn't rise in a linear fashion.
For some reason, TEPCO's Japanese page that has the handouts does not have the April 12, 2012 handout.
Unless TEPCO's photos and videos were fake the other day, the fuel rods inside the Reactor 4 Spent Fuel Pool are completely submerged under water (6.8 meters deep to the top of the fuel rods according to the diagram).